Why inspect hierarchy before applying SDC?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
SDC object paths depend on the post-synthesis hierarchy and naming that ICC2 actually loaded.
Technical Explanation
Hierarchy is the path used to find an object inside the design. Synthesis can rename or remove paths, so an old SDC selector may no longer point anywhere.
What To Check
• Warning sign: Constraints target pre-synthesis paths or a renamed block and resolve to nothing. • Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it. • Correct: Update the constraint producer's mapping or handoff for the real netlist; avoid broad wildcards as a shortcut.
Command Checks & Actions
• get_cells -hierarchical *: selects design cells
• current_block: confirms the active design block
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: SDC object paths depend on the post-synthesis hierarchy and naming that ICC2 actually loaded. • Investigate: Constraints target pre-synthesis paths or a renamed block and resolve to nothing. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Update the constraint producer's mapping or handoff for the real netlist; avoid broad wildcards as a shortcut.
Common Mistake
Do not assume the check passed just because ICC2 continued: Constraints target pre-synthesis paths or a renamed block and resolve to nothing. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning: Constraints target pre-synthesis paths or a renamed block and resolve to nothing.
Practical Example
Symptom: Constraints target pre-synthesis paths or a renamed block and resolve to nothing. Corrective path: Update the constraint producer's mapping or handoff for the real netlist; avoid broad wildcards as a shortcut.
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